The field of art to which this invention pertains is the hydrocracking of a hydrocarbonaceous feedstock. Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline by hydrocracking a hydrocarbon feedstock derived from crude oil or heavy fractions thereof, for example. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.), usually at least about 50 percent by weight boiling above 371° C. (700° F.). A typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts within a hydrocracking reactor influence the yield of the hydrocracked products.
Traditionally, the fresh feedstock for a hydrocracking process is first introduced into a denitrification and desulfurization zone particularly suited for the removal of sulfur and nitrogen contaminants and subsequently introduced into a hydrocracking zone containing hydrocracking catalyst. Another method of hydrocracking a fresh feedstock is to introduce the fresh feedstock and the effluent from the hydrocracking zone into the denitrification and desulfurization zone. The resulting effluent from the hydrocracking zone is separated to produce desired hydrocracked products and unconverted feedstock which is then introduced into the hydrocracking zone.
In the latter case, the diameter of the desulfurization and denitrification zone must be sufficiently large to accommodate not only the fresh feedstock but also the entire effluent from the hydrocracking zone. In world class hydrocracking units the diameter of the vessel utilized for the desulfurization and denitrification reaction zone becomes very large. Some of the largest vessels utilized in hydrocracking processes are 6.1 meters (20 feet) in diameter with wall thickness of 0.46 meters (1.5 feet) and weighing up to 2000 tons. At some point the required size of this vessel becomes larger than can be constructed by the existing manufacturing facilities, and the transport of very large vessels, because of their weight and girth, become impossible to transport via conventional methods. When this occurs the only alternative is to utilize an entire second train which not only needs a desulfurization and denitrification vessel but also another hydrocracking vessel. With the addition of a parallel train the equipment count and complexity of the hydrocracking plant become much greater.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new hydrocracking methods which provide lower costs, ease of construction, higher liquid product yields and higher quality products.